The invention relates to methods and apparatus for detecting electric current using the Faraday effect.
Measuring electric current using the Faraday effect has been employed in the electric power industry to measure alternating currents on high-voltage transmission lines. Generally, the Faraday effect relates to the measurable changes in the polarization of light precipitated by its propagation through the magnetic field surrounding the current to be detected and measured. In order to isolate the light which is used to detect and measure the electric current using the Faraday effect such light is normally linearly polarized and propagated through a fixed optical medium so that the effect of the magnetic field upon it can be readily measured.
The Faraday effect is an induced circular birefrigence of a transparent material, where the induced circular birefringence is caused by a magnetic field. Accordingly, when a light wave is passed through a transparent material in the presence of a magnetic field, the magnetic field has the effect of rotating the plane of polarization of the light wave. If a transparent optical material were placed near an electric current the magnetic field surrounding the electric current would provide such a magnetic field. This provides a means of measuring electric currents using the Faraday effect. The method is generally described as placing an optical medium in close proximity to an electric current and, under controlled conditions, propagating a light wave through this optical medium. The action of the Faraday effect on the light wave as it passes through the optical medium and is influenced by the electric current's magnetic field potentially can provide a meaningful measurement of the electric current itself.
Linearly polarized light is used in these devices. In order to better understand the action of the Faraday effect is useful to consider the linearly polarized light as decomposed into two counter-rotating circularly polarized light waves of equal amplitude. In the absence of a magnetic field in an isotropic medium these left-hand and right-hand circularly polarized light waves travel at equal velocity. The two indices of refraction n.sub.l and n.sub.r for the left-hand and right-hand light waves are equal. The magnetic field creates a difference between n.sub.l and n.sub.r. One circular polarization travels faster than the other, and the net effect is a rotation of the linearly polarized light that is produced when the two circular light waves are recomposed.
The direction of polarization is rotated in response to the component of the magnetic field that is parallel to the direction of transmission of the light wave. The strength of the Faraday effect in a given non-ferromagnetic medium is measured by the Verdet constant, V. The Verdet constant expresses a proportionality between the angular amount of rotation, .theta., of the plane of polarization of the light, and the product of the magnetic field strength, H, with the distance, L, the light travels through the medium in the presence of the magnetic field.
The relationship in meters-kilograms-second (mks) units is given by Equation 1, Table 1.
The state of polarization of the light wave is changed by interaction with transparent material. The change in the state of polarization is caused by both the Faraday effect resulting from the magnetic field produced by the current J and other properties of the transparent material. Stress birefringence in transparent material is particularly important in causing an unwanted change in the state of polarization of a light wave as it passes through transparent material.
Optical materials are never perfectly homogenous. Anisotropy in the refractive index is termed birefrigence. Linearly polarized light is converted to elliptically polarized light as it passes through a birefrigent material, because the birefringence disturbs the light wave. Residual birefringence is present to a greater or lesser extent in all optical materials, and is due to the (usually small) thermal stresses present in the material. As will be discussed later additional stress birefringence results when a bulk optical material is subjected to stresses from temperature variations.
As illustrated in FIG. 1 birefringent transparent Faraday material (70) has a characteristic direction indicated by arrow (72) at entrance face (78). The characteristic direction of the transparent material is defined in terms of the influence of the transparent material on linearly polarized light passing through the transparent material in the absence of a magnetic field. When a linearly polarized light wave (74) incident upon the transparent material (70) has its direction of polarization (76), aligned with a characteristic direction (72) of the transparent material (70), then the light emerges from the transparent material (70) linearly polarized. When the direction of polarization of the linearly polarized light (74) makes an arbitrary angle with the characteristic direction (72) of the transparent material (70), then the light emerges from the exit face (79) of the transparent material (70) elliptically polarized.
A transparent material has a characteristic direction as a result of electrical anisotropy of the transparent material. The electrical anisotropy of the transparent material gives rise to principal dielectric axes of the transparent material, as disclosed more fully in the textbook by M. Born and E. Wolf, "Principles of Optics" Pergamon Press, Sixth Edition, 1985.
The Born and Wolf textbook points out that in the general electrical anisotropic case a material will have three characteristic directions corresponding to the three principal axes of the dielectric tensor. The characteristic directions of optical materials has been further explored by H. K. Aben in an article entitled "Characteristic Directions in Optics of Twisted Birefringent Media" (Journal of the optical Society of America A/Vol. 3, No. 9, September 1986, pages 1414-1421). Aben points out that for inhomogeneous birefrigent optical media there are always two perpendicular characteristic directions. Light entering the medium with its electric field vector aligned with either of these characteristic directions will emerge from the medium also linearly polarized. That is, Aben points out that the medium always possesses two mutually orthogonal characteristic directions. This anisotropy noted above manifests itself as a difference in the index of refraction for the electric field components of the light wave parallel to each direction. As a result of the differences in the indicies the electric field components of the light wave move at different velocities. Consequently it is common to refer to a "fast" characteristic direction or a "slow" characteristic direction in the optical medium depending on the relative velocity of the components of the light wave parallel to the directions.
A variety of technologies exist to exploit the Faraday effect for the purpose of electric current detection and measurement. One popular method has been to enhance the magnetic field arising from the current by capturing it with a ferromagnetic ring and concentrating it upon an optical medium. Such a device is taught in Casey, et al, U.S. Pat. No. 3,324,393, issued on June 6, 1967. Another such device is taught in Feldtkeller U.S. Pat. No. 3,980,949, issued Sept. 14, 1976. In this device rather than surrounding the current with a magnetic ring the target current is passed around a ferromagnetic device a number of times. The ferromagnetic device, similar to one element of a transformer, then concentrates the magnetic field through a bulk optical material. This device has the disadvantage of disturbing the electric current by the inductance of the ferromagnetic element. These early devices do not deal with the effects of stress birefringence in the optical medium.
A recurring, but not adequately solved, problem with such devices is that of separating the effect of the phenomenon of stress birefringence upon the optical medium within the Faraday effect electric current detector, particularly such stress birefringence which can result from temperature changes in the optical medium. As an example of why stress birefringence is an important problem, such Faraday effect devices can be used as instrument transformers to measure electric current. Such equipment frequently is positioned outdoors and in environments with extreme climatic conditions. In such cases the widely changing temperatures create within the optical medium a high level of stress birefringence. Such a variation of the stress birefringence creates a serious problem since it alters the light wave and distorts the signal produced by the Faraday effect. This compromises the results achieved in the Faraday effect current detection and measurement system. A number of devices teach methods of overcoming this problem.
Some devices teach methods and apparatus for measuring a variety of the parameters of the light propagating through the medium and, based upon systematic calculations, separating the effects of stress birefringence from the Faraday effect produced by the current to be measured. While such devices have been found to work with a reasonable degree of accuracy, they have the disadvantage of complexity and in some cases the uncertainty inherent in the cascading of calculations necessary to deal with the disturbance caused by the stress birefringence.
An early attempt at removing the effects of stress birefringence is taught by Jaeckiln, U.S. Pat. No. 3,707,321, Dec. 26, 1972. Jaecklin teaches splitting the selectively polarized analyzing light beam into two components whose relative orientations are adapted to very nearly cancel out what are presumed to be equal and opposite stress birefringent effects. This leaves the Faraday effect as the sole source of any polarization shift in the analyzing beam. However, Jaecklin's device can only cancel out the effects of a single, unchanging value of the birefringence. In addition, a beamsplitter in his device can itself alter the state of polarization of the lightwave.
Another attempt at Faraday effect electric current detection is taught in Sato, et al, U.S. Pat. No. 4,564,754, Jan. 14, 1983. Sato also teaches a unique configuration of optical elements which are useful in eliminating the effects of nearby currents from interfering with the desired Faraday effect. This is accomplished by surrounding the target current with an optical network and by providing for two light wave reflections at each turn of the optical components around the target current. Sato ignores the problems created by birefringence, but is useful as an example of Faraday effect electric current detectors and the related signal processing. The inventor has previously published an article (High Accuracy Faraday Rotation Measurements, Ulmer, Paper No. ThCC 21, Pages 288-291, 1988 Technical Digest of Optical Fiber Sensors, New Orleans, La., January 1988) describing the calculations and methods existing to ferret out the results of stress birefringence in Faraday effect current measurements. Additionally, the inventor has been issued letters patent (Ulmer, Hooper, U.S. Pat. No. 4,755,665, July 5, 1988) on an apparatus useful in incorporating these methods in achieving accurate Faraday effect measurements.
Ulrich et al, U.S. Pat. No. 4,255,018, issued Mar. 10, 1981, teaches an attempt to eliminate the effects of birefringence from the Faraday effect of the light passing through the optical medium. It teaches an optical material which is, in fact, a twisted optical fiber. The theory behind this device is that twisting the optical fiber tends to "swamp out" the linear stress birefringence around the optical loop. However, the axial stress created by the twisting is itself temperature dependent thereby creating a new source of signal variations in the design by Ulrich.
A good overview of the current state of Faraday effect sensors is provided in Faraday Effect Sensors: The State of the Art, G. W. Day and A. H. Rose, National Bureau of Standards, 1988. This includes a discussion of several techniques for eliminating the undesired effects of birefringence. With respect to bulk optical materials, the proposed solutions concern utilizing materials which are very stable over changes in temperature. Such materials, such as SF-57, are very costly, and difficult to polish.
A further means of reducing the magnitude of the birefringence within a coiled optical fiber is developed in Day and Rose. It is accomplished by annealing the fiber while looped. A problem with this technique is that it requires removal of the protective plastic buffer on the optical fiber. The bare glass fiber is exposed and is very fragile and difficult to work with. Annealing can also be used on bulk optical materials in order to reduce the absolute value of the birefringence.
What is required but not provided in the present state of the art is a simplified Faraday effect system of accurately detecting and measuring electric current which is not affected by stress birefringence.